Engines may be configured with direct fuel injectors (DI) for injecting fuel directly into an engine cylinder and/or port fuel injectors (PFI) for injecting fuel into an intake port of an engine cylinder. Fuel injectors often have piece-to-piece and variability over time due to imperfect manufacturing processes and/or injector aging, for example. Over time, injector performance may degrade (e.g., injector becomes clogged) which may further increase piece-to-piece injector variability. As a result, the actual amount of fuel injected to each cylinder of an engine may not be the desired amount and the difference between the actual and desired amounts may vary between injectors. Variability in fuel injection amount between cylinders can result in reduced fuel economy, increased tailpipe emissions, torque variation that causes a lack of perceived engine smoothness, and an overall decrease in engine efficiency. Engines operating with a dual injector system, such as dual fuel or PFDI systems, may have even more fuel injectors (e.g., twice as many) resulting in greater possibility for injector variability.
Various approaches estimate injector performance by correlating a pressure drop across a fuel rail coupled to an injector with a fuel mass injected by the corresponding injector. One example approach is shown by Surnilla et al. in U.S. Pat. No. 9,593,637. Therein, a fuel injection amount for an injector is determined based on a difference in fuel rail pressure (FRP) measured before injector firing and FRP after injector firing. Another example approach is shown by Geveci et al. in U.S. Pat. No. 7,523,743. Therein, rail pressure sensor inputs and engine speed sensor inputs are used to determine multiple pressure values at each tooth position over a single engine cycle. An average or mean of the multiple pressure values is then used to infer injector leakage.
However, the inventors herein have recognized potential issues with such systems. As one example, there may be data errors in sampling the fuel rail pressure due to pressure ringing in the fuel making for aliasing errors. In particular, pressure may ring in the fuel rail for a duration during and following a fuel injection event. Given an inward-opening fuel injector, when the pintle moves inward, it compresses the fluid behind the injector, raising the fuel pressure. When fluid begins to exit the injector, the pressure drops (due to effective bulk modulus). When the pintle closes, its abrupt closing triggers a pressure oscillation (water hammer) that decays exponentially. Sampling in the presence of noise causes variation on a signal that one expects to represent a mean value. When this signal noise has a strong particular frequency content, the resulting sampled signal, even when averaged, could vary significantly from a mean value. A sampled signal of an oscillating signal may appear to be a shifted DC level or an AC signal of a different frequency than either the signal or the sample rate. Hence, it is referred to as an aliased signal, appearing to be something it is not. In addition to aliasing errors, there may be errors due to electrical or pressure noise. Pressure or electrical noise is largely expected to be uncorrelated to the sample rate and thus tends to reduce with averaging. Further still, data errors may be caused due to a finite analog to digital (AtoD) resolution. AtoD converters can only detect discrete voltage levels, not a truly continuous circuit. Since the actual fuel mass (or volume) injected is determined as a function of the fuel pressure drop, even small errors in fuel pressure sampling can translate into large fuel mass errors, resulting in incorrect injector compensation.
In one example, the issues described above may be addressed by a method for an engine comprising: for an injection event, averaging fuel rail pressure sampled after a delay since an end of injector closing; learning an injector fuel mass error for each engine injector based on the averaged fuel rail pressure; and adjusting subsequent engine fueling based on the learned injector error. In this way, fuel rail pressure changes corresponding to a fuel injection event can be determined more reliably, allowing for improved injector balancing.
As one example, during engine fueling, fuel rail pressure may be sampled over the course of a number of injection events. Fuel rail pressure (FRP) may be sampled at a defined sampling rate which may be synchronous or asynchronous with engine events. Each sample may include a fuel rail pressure estimate and an associated engine angle/position. Samples collected during an injection event (for a given injector) may be discarded. In addition, samples collected for a calibrated threshold duration (e.g., 5 msec) after the injection ends may be discarded. Samples collected on both PIP edges are then buffered. Specifically, the same samples collected after the threshold duration and before the start of the subsequent injection event are averaged. This corresponds to an average pressure for the given injection event. By comparing this average pressure to a similarly calculated average pressure for an immediately preceding injection event, a pressure difference may be determined. An actual fuel injection volume corresponding to the pressure difference is then calculated. By comparing the actual injection volume to a commanded injection volume for the given injection event, an error for the corresponding fuel injector may be determined. By similarly determining injector errors for all engine fuel injectors, and comparing the corresponding errors for all the injectors, fueling may be adjusted so that all injectors have the same error, thereby balancing the injectors.
In this way, fuel rail pressures sampled for a defined duration after a fuel injector has closed on an injection event are discarded. The technical effect of discarding samples in a noisy region of the sensor signal is that injector aliasing errors caused by pressure values sampled during a decay of pressure ringing can be removed. By only averaging fuel rail pressures sampled over quiet period of the fuel injection, (e.g., only between and the decay of the pressure ringing and the beginning of the next injection event), resolution errors are also reduced. As a result, fuel rail pressures and corresponding fuel injection volumes for fuel injectors can be estimated more accurately and reliably. This allows for improved injector balancing.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.